<template>
  <div ref="viewBox" class="abc">
    <div :class="{ fixed: true, bg: isColor }">我是头部</div>
    <div class="cont"></div>
  </div>
</template>
<style >
.fixed {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 15;
  color: #fff;
  text-align: center;
  font-size: 30px;
  line-height: 60px;
  width: 100%;
}

.bg {
  background: #000;
}

.abc {
  height: 100vh;
  overflow: auto;
}

.cont {
  height: 2000px;
  background: yellowgreen;
}
</style>
<script>
export default {
  data() {
    return {
      // box:{}
      isColor: false,
    };
  },
  methods: {
    scroll_top() {
      let _this = this;
      _this.box = this.$refs.viewBox;
      console.log(this.box);
      _this.box.addEventListener("scroll", () => {
        // _this.scrollTop = this.$refs.viewBox.scrollTop;
        console.log(this.$refs.viewBox.scrollTop);
        if (this.$refs.viewBox.scrollTop > 80) {
          this.isColor = true;
        } else {
          this.isColor = false;
        }
      });
    },
  },
  mounted() {
    this.scroll_top();
  },
};
</script>
